What is zygote?

What would be the ratio of chromosome number between an egg and its zygote? How is the sperm genetically different from the egg?

What is the term used for the process of development of mature embryo from zygote ?

In flowering plants, zygote is formed

Suppose there are 20 chromosomes in zygote of a plant and its female gametophyte develops aposporously. Now this plant is crossed with a normal male plant. What will be the number of chromosomes in its zygote and endosperm respectively ?